1. Field of the Invention
This invention relates to a method of amplifying an analog low-frequency signal by a switching amplifier which contains a number of switching stages which can be actuated independently of one another and the outputs from which are connected to a lowpass filter.
2. Description of the Prior Art
The amplification of analog signals by a switching amplifier may, in spite of the necessary conversion of the analog control signal into a control signal in pulse form and the conversion of the amplified pulsed output signal back into an analog output signal, be advantageous because all of the steps of the method may be performed with modern solid-state components, which in general permits a more compact construction and a longer working life of the amplifier as well as a considerable reduction in the stray power.
In the case of the methods known hitherto the switch members in the switching stages are actuated by pulse trains, the pulses in which exhibit a constant repetition rate and are width-modulated in accordance with the converted analog signal. Preferably two pulse trains are generated, the pulses in which are shifted in phase by 180.degree..
Such a switching amplifier is described, for example, in British Pat. No. 1,248,209 (Plessey Co. Ltd.). In the case of this amplifier the amplitude-modulated analog input signal is converted into a pair of pulse trains having width-modulated pulses shifted in phase by 180.degree.. Each pulse train controls one of the two switching transistors which are connected to the ends of the primary winding of a pulse transformer. The center tapping of this primary winding is connected to a source of supply voltage so that the pulse transformer is excited in push-pull. The secondary winding of the transformer is connected via a bridge rectifier to a lowpass filter at the output from which appears an analog signal which corresponds with the amplified input signal.
The output power from this switching amplifier is limited by the power which can be transformed by the one pulse transformer, and during amplification the pulses are deformed, wherefore the possibilities of employment of this switching amplifier are restricted.
That is why a switching amplifier has already been proposed (Swiss Patent Application No. 7307/79) which contains a plurality of switching stages for raising the switching power. In the case of this switching amplifier the analog input signal is converted into at least one pair of pulse trains, the two pulse trains shifted in phase by 180.degree. consisting of width-modulated pulses of constant repetition rate. Each of the switching stages contains two switching channels independent of one another, and each switching channel contains one pulse transformer, the primary winding of which is connected to one switch member. The one pulse train of the pair of pulse trains is provided for the control of the switch members in the first switching channels of the switching stages and the other pulse train is provided for the control of the switch members in the second switching channels. The secondary windings of all of the pulse transformers are connected in series for summing up the pulses transformed. The one output from each secondary winding is connected via a rectifier to the series lead and a further rectifier is provided in the series lead between the connections of the two outputs from each secondary winding.
In the case of a practically tested embodiment of this switching amplifier the switch members in 48 switching stages were controlled by each pulse train. The supply voltage for the primary windings of the pulse transformers was 500 volts and the transformation ratio selected as 1:1.2, so that at the end of the series circuit of the secondary windings of the transformers, amplified pulses were obtained having a peak voltage of up to 28 kV.
The insulation required between the secondary and primary windings of the pulse transformers forms at the aforesaid peak voltage an undesirable capacitance which is charged up and discharged at each switching or transformation process. The reversal of charge is connected with a relatively high stray power and at high switching frequencies may even bring about a deformation of the pulse transformed and thereby a distortion of the amplified analog signal.